1. Field of the Invention
The present invention relates to a fly-eye lens device incorporated in a projection printing and the like for use in a process of manufacturing LSIs and a lighting system including the fly-eye lens device.
2. Description of the Background Art
FIG. 6 is a schematic structural view of a projection printing which has been conventionally used in a photolithography process. FIG. 7 is a front elevation of a conventional fly-eye lens device incorporated in the projection printing.
As shown in FIG. 6, the projection printing comprises a lighting system 2 for uniformly illuminating a photomask 1. The lighting system 2 includes a light source 4 having a reflector 3; first optical means 7 composed of a lens 5 and a mirror 6; a fly-eye lens device 8; a stop 9; and second optical means 13 composed of lenses 10, 11 and a mirror 12.
The fly-eye lens device 8, as shown in FIG. 7, comprises a fly-eye lens 15 formed by two-dimensionally arranging a plurality of fly-eye constituent lenses 14 of square cross section and of the same size, and supporting means 16 such as an outer frame for supporting the fly-eye lens 15. The fly-eye lens device 8 is disposed such that the center of the fly-eye lens 15 coincides with that of an aperture 9a of the stop 9. A dashed-and-dotted circle P in FIG. 7 indicates the position of the aperture 9a of the stop 9 relative to the fly-eye lens device 8. For convenience, fewer fly-eye constituent lenses 14 are illustrated in FIG. 6 than the actual ones, and the illustration of the supporting means 16 is omitted in FIG. 6.
In FIG. 6, reference numeral 17 designates a projection lens, and 18 designates a semiconductor wafer.
In the projection printing, light L1 emitted from the light source 4, after passing through the lens 5 and reflected from the mirror 6, is introduced to the fly-eye lens device 8. The light impinging on the respective fly-eye constituent lenses 14 of the fly-eye lens device 8 passes through the aperture 9a of the stop 9, the lens 10, the mirror 12 and the lens 11 to illuminate the whole surface of the photomask 1. In FIG. 6, light L2 emanating from one of the fly-eye constituent lenses 14 which is disposed in the center thereof illuminates the whole surface of the photomask 1 by means of the second optical means 13. The lights emanating from the other fly-eye constituent lenses 14 similarly pass through the second optical means 13 to illuminate the whole surface of the photomask 1. Even when the light L1 emitted from the light source 4 has non-uniform light intensity distribution, the lights L2 emitted from the respective fly-eye constituent lenses 14 overlap each other to be averaged on the photomask 1, thereby uniform illumination being achieved thereon. For reference, L3 designates light converged on the photomask 1 by means of the second optical means 13 after emitted from the respective fly-eye constituent lenses 14.
Light L4 which has passed through the photomask 1 is directed through the projection lens 17 and converged on a resist film on the semiconductor wafer 18, so that a predetermined mask pattern is transferred thereto.
This type of projection printing, in some cases, employs the photomask 1 using what is called a phase shift method in order to improve the transfer accuracy of the mask pattern. In this case, when the size of the aperture 9a of the stop 9 is decreased, the coherence a of the light passing through the aperture 9a is increased, and resultingly a preferable contrast image as a mask pattern is expected. However, in the conventional projection printing, all of the fly-eye constituent lenses 14 of the fly-eye lens device 8 are of the same size (for example, the side of the square cross section of the fly-eye constituent lenses 14 is 5 mm in length) . When the size of the aperture 9a of the stop 9 is decreased, the number of fly-eye constituent lenses 14 which contribute to the uniform illumination is decreased. As a result, there has been a problem that non-uniform illuminance distribution on the photomask 1 causes the transfer accuracy of the mask pattern to deteriorate.
This problem is described in more detail below. It is commonly known that the coherence a is suitably 0.3 where the phase shift method is employed. In order to achieve the coherence .sigma. of 0.3 in the above-mentioned projection printing, the aperture 9a of the stop 9, for example, as shown by the two-dot chain circle Q of FIG. 7, must be smaller-sized than the conventional one (shown by the dashed-and-dotted circle P of FIG. 7). As a result, the number of fly-eye constituent lenses 14 which contribute to the uniform illumination is remarkably decreased (for example, it is reduced to approximately four in the example of FIG. 7). The illuminance distribution on the photomask 1 becomes non-uniform.
To solve the above-mentioned problem, it can be considered to form the fly-eye lens 15 with small-sized fly-eye constituent lenses 14. However, a large number of fly-eye constituent lenses 14 are required therefor. In general, the difficulty comes in processing and assembling a small-sized fly-eye constituent lens 14. It is not only difficult but also costly in terms of fabrication to form the fly-eye lens 15 only with the small-sized fly-eye constituent lenses 14.